1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for practicing a high-speed imaging method.
2. Description of the Related Art
A high-speed imaging method with a short repetition time TR, called a FLASH (fast low angle shot) method, is known. In this method, an MR echo signal is acquired by a field gradient method, and an RF pulse having a small flip angle .alpha. is used.
Although the contrast of the image can be changed to some degree by changing a flip angle .alpha. and the repetition time TR, it is difficult to obtain sufficient contrast.
Recently, therefore, high-speed imaging methods capable of changing the contrast more freely have been developed. One of such methods is disclosed in "Snapshot FLASH MRI. Applications to T1, T2, and Chemical-Shift Imaging", A. Haase, Magnetic Resonance in Medicine 13, pp. 77-89 (1990). In this snapshot FLASH method, the repetition time TR is extremely shortened (e.g., 3 ms), and the number of encoding steps (the number of pixels of an imaging matrix) in the phase encoding direction is reduced to 64, so that the time required for one imaging operation is set to be as short as 200 ms which is shorter than a spin-lattice relaxation time T1 of a tissue. This magnetic resonance imaging method is known to be capable of T1 enhancement, T2 (spin-spin relaxation time) enhancement, and chemical-shift-selective (CHESS) imaging as well as having a short imaging time.
In the snapshot FLASH method, however, the repetition time TR is set to be several ms which is extremely short as compared with general high-speed imaging, and a selective RF pulse and a slice selective gradient field are applied to perform selective excitation for each imaging operation. For this reason, a gradient field needs to rise and fall within a very short period of time, and the minimum repetition time TR (about 3 ms) is determined by the specifications of a gradient amplifier. The repetition time cannot be shorter than about 3 ms.